The present invention relates to a photometric circuit for a camera or the like.
With the spread of automatic cameras, it has become necessary to convert information concerning the luminance of a scene to be photographed into a digital signal and there has been proposed a digital photometric circuit as shown in FIG. 4, arranged such that a photoconductive element Rt for detecting the luminance of a scene to be photographed and a capacitor C are connected in series and the number of clock pulses generated during the period from the time when photometry is started until the terminal voltage of the capacitor C reaches a reference voltage is counted, as disclosed, for example, in U.S. Pat. No. 4,730,202.
Above described arrangement requires the capacitor C to have a large capacitance, e.g., about 0.22 .mu.F, and therefore suffers from the problems that it is difficult to fabricate the circuit in the form of an integrated circuit and the cost of the capacitor C is high.
The prior art also has the problem that the measuring accuracy may be lowered due to fluctuations in the electrostatic capacitance of the capacitor C or fluctuations in the reference voltage.
Further, as the luminance of the scene to be photographed becomes higher, the capacitor is charged more quickly and hence the measuring accuracy lowers. Since the degree of accuracy on the high-luminance side is relatively low due to the mechanical structure of cameras, the accuracy is synergistically lowered.
The prior art further has the problem that the comparator which is used as a means for making a comparison with the reference voltage needs a power supply which is capable of supplying a high voltage, i.e., three times the threshold voltage of each of the three transistors T.sub.1, T.sub.2 and T.sub.3 because these transistors are interposed in series with respect to the power supply Ve, as shown in FIG. 4.